|MadSci Network: Evolution|
Menstrual cycle is present in all Primates. It exists in other mammals (except egg laying mammal) as estrus cycle. In rodents its a roughly 4 day cycle. Reproduction (producing progeny) is a life time "hereditary investment". Reproductive success is evolutionary success. Menstrual cycle in human female is a monthly "house cleaning" activity. The uterus which houses the newly developing young one should be clean - free from viral, bacterial, fungal and other parasitic infection and infestations. Menstrual cycle ensures this - by mechanically eroding the top layer of tissue (endometrium) on a monthly basis.
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